How can add Android 6.0 on my API Level ( I have all my sdk installed up until Android 7.0 Nougat)

I believe Unity 5.4 doesn’t support that API Level, as it’s a bit old.
But Unity 5.5.0f1 and newer definitely support API Level 23 (Just checked it)
Also, Unity 5.6 (Beta) supports API Level 24 and 25 (Nougat 7.0 & 7.1)